This invention relates generally to acceleration physiology, and more particularly to apparatus which can objectively measure by non-invasive means the arterial blood volumetric flow of a human in a rapidly accelerating vehicle.
Under acceleration, especially in the +G.sub.z or headward direction, a human will experience a wide variety of physiological changes among which is a reduction in perfusion blood pressure to the head area, particularly to the brain and eyes. Such a reduction occuring gradually results first in a loss of peripheral vision ("greyout"), followed by a complete loss of vision ("blackout"), and ultimately in a loss of consciousness. During rapid onset of acceleration, however, perfusion blood pressure to the head area is suddenly reduced resulting in a loss of consciousness without the typical "greyout" or "blackout" warning signals. This situation may occur, for example, to an occupant of a high performance aircraft capable of routinely exceeding 6 G's per second or in centrifuges which are specially adapted to produce centripetal accelerations with onset rates similar to in-flight environments.
Subjective endpoint criteria such as "greyout" and "blackout" have long been used in the evaluation of physilogical anti-G devices. Nevertheless, because of the variability between test subjects with regard to experience, training, or motivation, and differences between testing facilities and testing criteria, such subjective measurements have generally been abandoned in favor of objective tests. On rare occasions, invasive pressure catheters have been utilized to measure arterial blood pressure response to acceleration. Due to the inherent danger for the human test subject involved and the compounding of these dangers by the acceleration, however, such invasive techniques are generally no longer used.
Perhaps the most popular device currently being used at centrifuge test facilities to objectively measure cardiovascular status during human acceleration stress studies is the transcutaneous, ultrasonic Doppler velocimeter. In most meters of this type, the Doppler frequency shift has been weighted using a power frequency technique so that it is proportional to the mean velocity. Examples of such meters and their use in acceleration physilogy may be found in Rositano et al., "Non-Invasive Determination of Retrograde Eye-Level Blood Flow as a G.sub.z Tolerance Indicator, Proceedings of the 44th Annual Scientific Meeting, Aerospace Medical Association, Las Vegas, Nev., May 7-10, 1973.
Numerous methods which utilize such ultrasonic Doppler velocimeters have been proposed in the past for converting the blood velocity signals received thereby into the more useful blood volumetric flow information. With the majority of these techniques, however, it is assumed that the cross-sectional area of the blood vessel under examination remains constant throughout the cardic cycle. While such an assumption is considered valid for normal physilogical conditions where the radius of an artery varies by only 7 to 10 percent, it becomes extremely restrictive under adverse conditions such as those experienced during acceleration stress where the decreasing pressures in arteries supplying the head can lead to reductions in the flow cross-sectional area of between 48 and 64 percent. Since the flow rate Q is equal to the product of mean arterial velocity V and the cross-sectional area A, significant errors in the calculated flow can be introduced under the constant cross-sectional area assumption.